The present invention relates in general to electrical-test equipment used for tracing conductors and identifying electrical circuit elements. More particularly, the present invention relates to test equipment for identifying a circuit interrupter (e.g., circuit breaker or fuse) that is associated with a particular branch circuit of an electrical wiring system.
The electrical systems of homes, offices and other buildings consist of a plurality of branch circuits that originate at one or more power distribution panels. To protect against overload currents, each branch circuit includes a circuit interrupter (circuit breaker or fuse) that is installed in the power distribution panel associated with that particular branch circuit. Thus, the power distribution panels typically include a number of circuit interrupters. Most electrical distribution systems provide single or three phase AC power. However, some buildings, boats and ships employ DC power distribution systems or distribution systems that include both AC and DC branch circuits.
Situations arise in which it is necessary to locate the circuit interrupter that is associated with a particular AC or DC branch circuit. For example, when electrical work that is associated with a particular branch circuit is to be performed, it is usually necessary to interrupt electrical power to that branch circuit without interrupting power to other branch circuits. It is common practice to include legends that associate each circuit interrupter with a branch circuit when an electrical system is first installed or one or more branch circuits are added. For example, many power distribution panels include a hinged cover plate having a diagram of the layout of the circuit interrupters and a space for handwritten identification of the area, appliance or other electrical load that is served by that branch circuit. However, it is not unusual for the written legends to be illegible because of the passage of time or poor penmanship. Additionally, in some cases, the legend may not be specific enough to clearly identify the branch circuit of concern or, in some cases, may be incorrect.
Various methods and devices have been developed to avoid the tedious and often undesirable procedure of manually tripping circuit breakers (or removing fuses) until power is not supplied to an electrical outlet or socket that is located in a branch circuit that is to be de-energized.
One type of device that has been developed for locating circuit interrupters includes a relatively small transmitter unit and a handheld receiver. The transmitter unit is plugged into an electrical outlet or installed in a socket that is located in the branch circuit that is to be traced to a circuit interrupter. In many cases, the transmitter is a relaxation oscillator in which a semiconductor device such as a voltage controlled switch (e.g., a SIDAC) or a semiconductor circuit such as a diac triggered thyristor is switched to a conductive state to rapidly charge a capacitor. The rapid charging of the capacitor causes a current spike of relatively short duration to propagate through the branch circuit to which the transmitter is connected. As current through the capacitor decreases, the switch circuit resets and the capacitor discharges through a current path that is provided within the transmitter. Thus, the transmitter periodically induces current pulses in the associated branch circuit with the pulse repetition rate being determined by the RC time constant of the capacitor that induces the current spike and the resistance of the capacitor discharge path.
The receivers that are used in the transmitter-receiver arrangements for locating circuit interrupters typically are battery powered devices that include a sensor coil (“pickup coil”) and usually are broadly tuned for receiving signals at the transmitter pulse repetition rate. In operation, the handheld receivers are placed in close physical proximity to the electrical system circuit interrupters so that maximum electromagnetic coupling occurs between a selected circuit interrupter and the receiver pickup coil. Typically the receiver includes a variable gain stage and other circuitry that drives audible and/or visual indicators (such as a piezoelectric beeper and/or light emitting diode (LED)) that are activated in response to received current pulses.
Power distribution panels position the branch circuits and circuit interrupters in relatively close proximity to one another. Thus, a current pulse induced in a particular branch circuit is electromagnetically coupled to closely proximate branch circuits and to associated circuit interrupters. Moreover, current fluctuations may be present in branch circuits other than the one being traced. As a result, the audible and/or visual indicators of a receiver may be energized when the receiver pickup coil is positioned adjacent more than one of the circuit interrupters. One technique that has been used to overcome multiple and false receiver indications is to manually decrease the gain of the receiver variable gain stage until a single circuit interrupter is identified. Other techniques that have been used to reduce false receive indications include using a transmitter pulse repetition rate significantly lower than the frequency of the AC branch current to minimize interference from light dimmers and other devices and using transmitter pulse repetition rates that are located between harmonic frequencies of the AC branch current.
Although arrangements for locating circuit interrupters associated with particular branch circuits of an electrical distribution system have met with a degree of commercial success, a need exists for improved reliability and ease of use performance so as to minimize the presence of false indications.